1. Field of the Invention
The present invention relates to an image-forming apparatus as represented by a copying machine, a printer, a facsimile, a compounding machine or the like machine. More specifically, the invention relates to an image-forming apparatus equipped with an endless belt which runs being tightly stretched round plural rolls maintaining a high degree of correctness.
2. Prior Art
An image-forming apparatus such as a copying machine or printer is employing a belt running device in which, as shown in FIG. 18, an endless belt 200 runs in a direction of an arrow e being tightly stretched round plural rolls 101, 102 and 103, an image (toner image) formed on a photosensitive drum or the like by the electrophotographic system is once transferred onto, and carried by, the surface of the endless belt 200, and is transferred again onto a recording sheet such as recording paper which is conveyed so as to meet the belt 200, so that the belt running device works as a so-called intermediate transfer belt mechanism.
It has further been known to use the above belt running device as a photosensitive belt mechanism in which an electrostatic latent image corresponding to the image data and the toner image thereof are formed on the surface of an endless belt by the eletrophotographic system, or as a sheet conveyer/transfer belt mechanism in which a recording sheet onto which the image will be finally transferred is held by being adsorbed by the surface of the endless belt and is conveyed to a transfer position of a photosensitive material drum or the like on which the image is formed, and the recording sheet onto which the image is transferred from the photosensitive material drum or the like is discharged.
In the image-forming apparatus which utilizes the above belt running device, however, there occurs a zigzag phenomenon in which an endless belt 200 runs being shifted in the axial direction of the rolls (corresponds to the direction of width of the belt) or the endless belt 200 runs in a tilted manner being deviated from the direction at right angles with the rolls 101, 102 and 103, due to error in the production of belt and rolls for supporting the endless belt, error in the parallelism of rolls, and difference in the tension given to the belt at an end and the other end of each of the rolls. Thus, there often occurs a twisting phenomenon in which the endless belt 200 as a whole runs in a state of being twisted like a "figure 8" as shown in FIG. 19. When the endless belt runs in a state where the zigzag phenomenon or the twisting phenomenon is occurring, the image is not transferred onto a normal position on the endless belt or on the recording sheet in the case of the zigzag phenomenon. Or, in the case of the twisting phenomenon, a distorted image is transferred from the photosensitive drum onto the endless belt which is the intermediate transfer belt, or the image transferred again onto the recording sheet after once transferred onto the endless belt is skewed to a large extent. The above problems induce a new problem in that the image quality becomes defective due to deviation in the colors, color shade and the like in the color image formed by overlapping toner images of plural colors.
In order to solve such problems, the present applicant has proposed an image-forming apparatus (Japanese Patent Laid-Open No. 260590/1998, etc.) which is so constituted that, when the endless belt is running in a zigzag manner or in a twisted manner, an end of the drive roll for supporting and running the endless belt is displaced upward or downward, or an end of a back-up roll supporting the endless belt at a secondary transfer position where the image is transferred onto the recording sheet is displaced upward or downward by using an eccentric cam mechanism or the like, in order to incline the roll as a whole.
However, when one of the rolls supporting the endless belt is so displaced as to be inclined upward or downward, the tension of the endless belt undergoes a change, resulting in the occurrence of a problem that the belt is twisted due to a change in the tension. Here, the change in the tension is as shown in FIG. 18b; i.e., when an end of the roll 101 is inclined upward (white arrow) Yu, the tension applied to the endless belt portion 200a on the side inclined upward becomes larger than that of normal condition and, conversely, when the end of the roll 101 is inclined downward (hatched arrow) Yd, the tension applied to the endless belt portion 200a on the side inclined downward becomes smaller than that of normal condition. This problem may similarly occur even in the image-forming apparatus proposed by the present applicant in the Japanese Laid-Open Patent Application No. 2000-075680.